The invention relates to a method for operating an ultrasound oscillation system comprising an ultrasound oscillator and components forming therewith an oscillation circuit, wherein for the generation of an excitation current an excitation voltage is applied to the ultrasound oscillation system, whose frequency is adjustable for the operation of the ultrasound oscillation system at a predetermined operating point, and, upon switching on the ultrasound oscillation system, starting out with a certain startup frequency, the frequency is changed until the operating point is reached.
Furthermore, the invention resides in a circuit arrangement for operating an ultrasound oscillation system according to the method referred to above.
Such a method and such a circuit arrangement are well-known in the state of the art and are used for example in connection with the ultrasound welding apparatus manufactured and sold by the assignee of the present application.
For ultrasound welding apparatus, it is necessary that the energy input into a particular work piece is constant. The ultrasound oscillation system needs to be constant. It is therefore important particularly in connection with ultrasound welding apparatus, that the oscillation amplitude of the ultrasound oscillation system is constant. This is because the energy input into a particular work piece depends on the oscillation amplitude of the welding head which means that the energy input into a particular work piece depends on the oscillation amplitude of the ultrasound oscillation system. Since the oscillation amplitude of the ultrasound oscillation system depends on the excitation current of the ultrasound oscillation system comprising an ultrasound oscillator and the components forming therewith an oscillation circuit, the oscillation amplitude of the ultrasound oscillation system is maintained constant in that the excitation current of the ultrasound oscillation system is kept constant.
In order to be able to control the excitation current, the ultrasound oscillation system is not operated at its series circuit resonance frequency, but generally at a frequency which is between the series circuit resonance frequency and the parallel circuit resonance frequency of the ultrasound oscillation system. Since by changing the frequency with which the ultrasound oscillation system is operated, the impedance of the ultrasound oscillation system is changed, the current flowing through the ultrasound oscillation system can be changed by changing the operating frequency of the ultrasound oscillation system.
If, during the operation of the ultrasound oscillation system, the current through the ultrasound oscillation system is changed, for example by external influences, the frequency of the excitation voltage applied to the ultrasound oscillation system is changed until the excitation current of the ultrasound oscillation system has again reached the previous value.
For reaching the operating point, the frequency of the excitation voltage, beginning at a startup value is changed until the excitation current has reached its predetermined value. The startup frequency is generally about 2 to 5 percent above the operating frequency of the ultrasound oscillation system and, consequently, also above a parallel resonance frequency. The relatively large distance of the startup frequency from the operating point of the ultrasound oscillation system is necessary to ensure that the startup frequency is above the operating frequency of the ultrasound oscillation system also when the operating point has changed for example as a result of a temperature event of the ultrasound oscillation system.
If the frequency of the excitation voltage is reduced, the impedance of the ultrasound oscillation system increases until the parallel resonance frequency is reached whereby the excitation current is reduced. When the parallel resonance frequency is exceeded, the impedance of the ultrasound oscillation system becomes smaller so that the excitation current increases. When the excitation current reaches its predetermined value the ultrasound oscillation system is at its operating point whereupon a control is initiated by which the excitation current is maintained constant.
Because of the relatively large frequency change of the excitation voltage during the switching on of the ultrasound oscillation system, the ultrasound oscillation system reaches its operating state only with a delay. The time delay until the ultrasound oscillating system reaches its operating state may be several hundred milliseconds. This is very disadvantageous since, as a result, an increased amount of time is required for a welding procedure and, consequently, the cycling time of an ultrasound welding apparatus is increased.
It is the object of the present invention to provide a method or, respectively, a circuit arrangement of the type referred to in the introduction in such a way that the time required for reaching the operating point is reduced.